Practice question

Classic fate mapping using carbon particles and fluorescent dyes shows chick epiblast before gastrulation is pluripotent sheet. Single labelled epiblast cells can contribute descendants to neural ectoderm, epidermis, all mesodermal subtypes including paraxial, lateral, extraembryonic and definitive endoderm depending on ingress timing through streak. Hypoblast only yields extraembryonic yolk sac endoderm that eventually replaced. Thus epiblast at stages X-XIII harbors progenitors for all three germ layers, reflecting epiblast as embryonic source homologous to amphibian animal and marginal zones combined, not restricted to only one layer.